I additionally know, and can prove, that the DPR studio scene samples offer no set of camera ISOs such as to meet your preferred criterion. However, they do offer one set of camera ISOs such as to meet another criterion that I find acceptable, namely one in which the cameras are given the same signal (as I defined the word signal above) although the mean raw values are not the same.

I don't think that they do do that. DPR exposes as I understand for a fix point in the sRGB output - that is that 18% (though they never specify 18%) on their step wedge gives the appropriate value in the JPEG file. You have to relate that back to the effective (meter ignored) ISO value that they discovered (and do not always report) in their ISO 'sensitivity' test.

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Bob

Those ISO values are generally the same (within 1/6th of a stop) though, aren't they. And they do report if a camera is outside that, such as the OM-D at 1/3 of a stop.

Correct! With regard to the deviation of 1/3 EV for the E-M5, see here: